A radio system for a closed user group is known in Germany by the name of "Chekker". This radio system comprises several base stations which serve one radio cell each and are connected to a main system controller via "trunked system controllers" (see, for example, FIG. 6-2/2 on p.6, Chapter 6.2, in the "Mobilfunk Handbuch" by J. Kedaj et al, Neue Mediengesellschaft Ulm m.b.H., Edition May 1993). Accordingly, the "Chekker" radio system, which is designed to provide private mobile radio service within service areas with radii of about 50 km, has a complex infrastructure. A connection between a private mobile radio unit and the public telecommunication network is only possible via the main system controller.
A radio system for a closed user group which serves a small, single-cell area is described in DE 43 07 966 A1. A so-called transmission station is provided there which is connected to the public telecommunication network via switching equipment. This transmission station is similar in construction to a base station and a base station controller and, thus, complex and costly. Furthermore, the protocol executed between the transmission station and a radiotelephone is different from a standard protocol, so that the transmission stations must incorporate transceiver technology which is not standardized and, thus, cannot be procured at low cost.